Physiology of Digestion

Question 1

What are the three things that carbohydrates are digested to?

Answer 1

Polysaccharides
Oligosaccharides
Monosaccharides

Question 2

What must carbohydrates be converted to for absorption?

Answer 2

monosaccharides

Question 3

Describe Polysaccharides?

Answer 3

Starch - two kinds
Amylose is a linear molecule where the individual glucose molecules are held together by alpha 1,4 bonds
Amylopectin is branched chain and linked by alpha 1,6 bonds

Glycogen (in plants) - branched chain α-1,4 and
α-1,6 linkages

Question 4

Describe Oligosaccharides?

Answer 4

Sucrose = Glucose + Fructose, α-1,2 linkages
Lactose = Glucose + Galactose ß-1,4 linkages

Question 5

Describe monosaccharides?

Answer 5

Glucose

Fructose

Question 6

What enzyme is involved in carbohydrate digestion?

Answer 6

α-Amylase

Question 7

What are the steps of carbohydrate digestion?

Answer 7

Intraluminal hydrolysis = starch to Oligosaccharides
Membrane digestion (at brush border) = Oligosaccharides to monosaccharides

Question 8

Describe alpha amylase?

Answer 8

Endoenzyme
breaks down linear internal α-1,4 linkages but not terminal α-1,4 linkages. Hence, no production of glucose

cannot cleave α-1,6 linkages at branch points (in amylopectin) or α-1,4 linkages adjacent to branch points
products are thus linear glucose oligomers (maltotriose, maltose) and α-limit dextrins

Question 9

What is the role of Oligosaccharides?

Answer 9

Oligosaccharidases are integral membrane proteins with a catalytic domain that faces the lumen of the GI tract

• Lactase has only one substrate – breaks down lactose to glucose and galactose
• All other oligosaccharidases cleave the terminal α-1,4 linkages of maltose, maltotriose and α-limit dextrins (to yield glucose)

Question 10

What is Sucrase responsible for?

Answer 10

for hydrolysing sucrose to glucose and fructose

Question 11

What is unique about Isomaltase?

Answer 11

enzyme that can split the branching α-1,6 linkages of α-limit dextrins

Question 12

What is the role of maltase?

Answer 12

can degrade the α-1,4 linkages in straight chain oligomers up to nine monomers in length

Question 13

What can lactose intolerance come from?

Answer 13

Primary lactase deficiency (primary hypolactasia) – due to lack of the lactase persistence (LP) allele – most common cause world wide
Secondary lactase deficiency – caused by damage to/ infection of/ the proximal small intestine
Congenital lactase deficiency – rare autosomal recessive disease – no ability to digest lactose from birth

Question 14

What are the consequences of lactose consumption in lactose intolerance?

Answer 14

If lactose is delivered to the colon from the ileum colonic, microflora produce:
short-chain fatty acids (which can be absorbed)
hydrogen (H2 - which can be detected in the breath of lactase deficient individuals following a lactose challenge)
carbon dioxide
methane
These by products produce: bloating, abdominal pain, flatulence

Question 15

What does undigestid lactose cause?

Answer 15

acidification of the colon

an increased osmotic load – loose stools and diarrhoea

Question 16

Describe the absorption of the Final Products of Carbohydrate Digestion: Glucose, Galactose and Fructose?

Answer 16

Occurs in the duodenum and jejunum
Is a two step process involving entry and exit from the enterocytes via the apical and basolateral membranes, respectively
Glucose and galactose are absorbed by secondary active transport mediated by SGLT1; fructose by facilitated diffusion mediated by GLUT5.
Exit for all monosaccharides is mediated by facilitated diffusion by GLUT2

Question 17

What are the rules for SGLT1 transport?

Answer 17

A hexose in the D-conformation

One that can form a pyranose ring

Question 18

Describe the mode of operation of SGLT1?

Answer 18

1. 2 Na+ binds:
2. Affinity for glucose increases, glucose binds:
3. Na+ and glucose translocate from extracellular to intracellular:
4. 2 Na+ dissociate, affinity for glucose falls:
5. Glucose dissociates:
6. Cycle is repeated

Question 19

How must a protien be absorped?

Answer 19

digested to oligopeptides and amino acids

Question 20

How are protiens converted to peptides?

Answer 20

luminal enzymes

Question 21

How many proteins digestion pathways are there?

Answer 21

4 - LOOK AT THE POWERPOINT AND LEARN THEM

Question 22

Describe digestion in the stomach?

Answer 22

HCl begins to denature proteins
Pepsin cleaves proteins into peptides
- has pH optimum of 1.8 to 3.5, inactivated at alkaline pH
- is an endopeptidase with preference for bonds between aromatic and larger neutral amino acids

Question 23

Describe digestion in the duodenum?

Answer 23

The five pancreatic proteases are secreted as proenzymes from the exocrine pancreas and converted to active form in the duodenum.
They function as either endopeptidases, or exopeptidases

Question 24

Describe the 5 pancreatic proteases?

Answer 24

Trypsin - Endopeptidase- product: Oligopeptides (2-6 amino acids)

Chymotrypsin - Endopeptidase - product: Oligopeptides (2-6 amino acids)

Elastase - Endopeptidase - product: Oligopeptides (2-6 amino acids)

Procaroxypeptidase A - Exopeptidase- product: Single amino acids

Procarboxypeptidase B - Exopeptidase - product: Single amino acids

Question 25

Describe Endopepitdases?

Answer 25

chemically attacks the bonds of amino acids within the polypeptide chains (not at the end) – hydrolysis product is then smaller polypeptide fragment

Question 26

Describe Exopetidase?

Answer 26

can cleave off a single amino acid

Question 27

What two peptidases make up exopetidase?

Answer 27

aminopeptidase

carboxypeptidase

Question 28

To complete protein digestion - what additional proteases help?

Answer 28

Additional proteases are present:
at the brush border
within the cytoplasm of the enterocyte

Question 29

Describe brush border peptidases?

Answer 29

numerous – because each enzyme attacks a limited number of peptide bonds and the oligopeptides to be digested are extremely varied in their structure (up to 24 different amino acids are present in protein)
have affinity for larger oligopeptides (3 – 8 amino acids)
act either as endopeptidases, or exopeptidases, the latter comprising both aminopeptidases and carboxypeptidases

Question 30

Describe cytoplasmic peptidases?

Answer 30

are less numerous than brush border peptidases

primarily hydrolyse dipeptides, or tripeptides

Question 31

How are di-,tri-, and tetrapeptides absorped?

Answer 31

via H+-dependent mechanism (PepT1, SLC15A) at brush border (co-transport)
further hydrolysed to amino acids within the enterocyte
Na+-independent systems at the basolateral membrane (facilitated transport)

Question 32

How are amino acids absorped?

Answer 32

Brush border – at least 7 different mechanisms are present
5 are Na+-dependent co-transporters mediating ‘uphill’ movement (secondary active transport)
2 are Na+ independent
Basolateral membrane – at least 5 different mechanisms: 3 mediate efflux of amino acids and are Na+-independent: 2 mediate influx